RADAR Flashcards
What are the principles of radar?
Radio Detection and Ranging.
Electromagnetic energy is radiated in the form of a rectangular pulse from a transmitter, an electronic timing device is started, the energy reflects on any obstructions, energy is re radiated back towards it’s point of origin. A receiver detects it and the timer is stopped. The range of the obstruction can be determined using the speed/distance/time formulas
What are the carriage requirements for radar?
Every vessel over 300GT or every passenger vessel require 1 x-band radar
Over 3000GT require 2 radars
>10,000GT 2 radars (1 must be x-band and the other must be s-band)
What are the differences between ATA and ARPA?
ATA. Automatic tracking aid. (Required on vessels greater than 500GT) - Can track up to 20 targets - Manual acquisition of targets - No trial manoeuvre function ARPA. Automatic radar plotting aid. (required on vessels over 10,000GT) - Can track up to 40 targets - Automatic target acquisition - Trail manoeuvre function
What are the 2 principle types of marine radar? Explain the difference
X-band radar. Transmits 3cm wavelengths at a frequency of 9GHz Advantages - Good definition. Good for small targets at close range - Detects SARTS - Smaller antenna - Less sea clutter adjustment required Disadvantages - Suffers from rain clutter - Shorter range - Sea clutter more evident S-band radar. Transmits 10 cm wavelengths at frequency of 2.4 GHz Advantages - Long range - Less sea clutter evident - Less affected by rain clutter - Good for detecting ice Disadvantages - Suffers from promulgation errors - Requires high sea clutter settings - Scanner very large - Suffers large target spread
What is a SART?
Search and rescue transponder.
Required on all vessels over 300GT and all passenger vessels. Over 500GT 2xSARTS
Battery life 96 hours on standby and 8 hours in transmit mode.
The SART will be detected by an x-band radar. Initially there will be 12 dots on the bearing of the SART. As the range decreases, the dots will turn to arcs. Withing about 1NM the arcs will become concentric circles.
For monthly maintenance use designated test button. Ideally carry out test when there are no vessels in the vicinity. However if necessary, broadcast a Safety message to vessels in the vicinity.
Describe how you would start the radar?
- Check mast is clear. Check for permits to work
- Power on the radar. Allow to heat up.
- Check mast again
- Switch to transmit
- Brilliance. Turn up the brilliance until the sweep is visible
- Range. Turn to the appropriate range. Either 6 or 12 for setup.
- Gain. Turn up until a light speckled background. Then turn back very slightly.
- Tune. Setting the frequency of the signal. Automatic Frequency Control (AFC)
- Sea clutter. Works from the inside out. Adjust the sensitivity of the receiver.
- Rain clutter. Works from the outside in. Suppresses weak echoes.
What are the controls of the radar?
Range. From 0.25 to 24 NM
Pulse length. The selected range scale determines the pulse length which, in turn, determines both the pulse repetition frequency (PRF) and the pulse repetition interval (PRI). The length of the pulse must be such that the receiver can distinguish between targets on the same bearing.
Brilliance. The display should be adjusted to provide a clear picture according to the prevailing conditions
Gain. This is the term used to mean amplification of the received signal. Too little gain and targets at the edge of the screen may not be displayed. Too much gain and the targets begin to merge with one another
Tuning. The majority of commercial radars are fitted with automatic frequency control (AFC) to obtain the optimum intermediate frequency for video processing.
Sea clutter. This control applies a suppression to the amplifier, the effect of which varies with distance from the transmitter
Rain clutter. the transmitted pulse is broken up into a series of shorter pulses. The much shorter transmitted pulses have insufficient energy to be reflected by individual rain drops but will re-combine (and are reflected) on striking a solid object.
How would you set up the radar for collision avoidance? And for navigation?
Collision avoidance • Course up • Relative motion • Relative trails • True vectors • Sea stabilised navigation • • North up • True motion • True trails • Relative vectors • Ground stabilised
What are the limitations of radar?
- Range and bearing discrimination
* False echoes
What are the limitations of ARPA?
- The time it takes. It take 3 minutes of steady state tracking to produce accurate readings.
- Target swap. When 2 targets get close to one another or when multiple targets appear.
- Inputs. Heading and speed. Inaccurate or missing data.
What are the requirements of ARPA?
Within 1 minute of steady state tracking, should give a trend with CPA Accuracy of 2 NM.
Withing 3 minutes of steady state tracking should give predicted motion with CPA accuracy of 0.5 NM
A.823(19) Performance standards of ARPA.
What is range and bearing discrimination?
Range discrimination. Ability of the radar to display as 2 targets the echoes from 2 objects on the same bearing but separated by a small distance. Caused by the pulse length relative to the range.
Bearing discrimination. Ability of the radar to display as 2 separate targets the echoes from 2 objects at the same range but on similar bearings. Caused by a horizontal beam width.
What non-standard conditions can affect the radar image?
Sub-refraction. Reduction in radar range caused by abnormal decrease in temperature with height, and increase of relative humidity. most prevalent in high latitudes where a cold air mass flows over a warmer sea.
Super-refraction. Increase in radar range caused by temperature falling more slowly than usual with height, and decrease of relative humidity with height. occurs mainly in temperate latitudes where a warm, dry, air mass flows from the land over a warm sea.
Ducting. Where super-refraction occurs at the same time at which there is a temperature inversion in the upper atmospheric layers, the radar beam may become ‘trapped’ in a duct. On reflection from the upper inversion layer, the beam is again attenuated to follow the curvature of the earth.
What is the minimum range of the radar?
Affected by Pulse length, T/R cell changeover, Vertical beam width, Physical obstructions, Height above sea level
What are the different false echoes on a radar?
Indirect echoes. These are caused by reflections from obstructions in the path of the radar beam. They generally only appear in the blind or shadow sectors generated by masts, funnels, Samson posts etcetera.
Multiple echoes. These are caused by a radar pulse being reflected backwards and forwards between two reflecting surfaces before being received at the scanner. This most frequently occurs when passing a large target, presenting good aspect, at close range. The result is a series of progressively weaker but equally spaced echoes on the same bearing,
Second trace echoes. In certain extreme conditions, the return from one pulse may be received at the scanner after the next pulse has already been transmitted. The receiver then processes this echo as if it had been transmitted by the second pulse and so will paint an echo at a false range
Side lobe echoes. Small amounts of energy from each pulse escape at an angle to the perpendicular and create small lobes each side of the main transmission pulse
Interference. transmissions from another radar in the vicinity which is transmitting on an identical (or very similar) frequency and using the same pulse repetition frequency as own vessel. It is usually associated with a series of spirals
Ghost echos. Generated by the electro-magnetic fields surrounding power cables. the appearance on the radar screen is that of a target on a steady bearing
